The following discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was published, known or part of the common general knowledge in any jurisdiction as at the priority date of the application.
Second harmonic generation (SHG) microscopy is a powerful imaging tool for disease diagnosis and envision of tissue engineering scaffolds. Second harmonic (SH) probes have been developed for in-vivo imaging in recent years as they possess many advantages over the classic fluorescent probes. SH probes are typically made of inorganic nanomaterials, such as barium titanate nanocrystals. Although these materials have achieved certain success in molecular imaging, they are concerned for their health and environmental issues.
In addition, second-harmonic generation (SHG) microscopy is a nonlinear optical (NLO) imaging technique that has emerged in recent years to visualize structures or functions of cells, tissues and organisms [1]. It has the advantages of being label-free, inherent three-dimensional (3D) resolution, near-infrared (NIR) excitation for superior optical penetration, lower photo-damage, and capable of providing quantitative information, which makes it an attractive tool for high resolution imaging [1-3]. It has been used to diagnose a wide range of diseases including cancer, liver fibrosis and skin damage [4]. It has also been used in tissue engineering and regenerative medicine [5] to envision three-dimensional biomaterial scaffolds and extracellular matrices that secreted by cells [6-7].
SHG is an optical frequency doubling process: when a non-linear optical (NLO) material interacts with light, two NIR incident photons can be converted into one emerging visible photon at exactly twice the energy (or half the wavelength) [1,4,9]. This NLO material requires a noncentrosymmetric structure. Examples include potassium niobate (KNbO3), lithium niobate (LiNbO3), barium titanate (BaTiO3), potassium titanyl phosphate (KTiOPO4, KTP), and zinc oxide (ZnO), Gallium nitride (GaN), Silicon carbide (SiC), noble metal particles, and quantum dots [8,9].
Inorganic nanomaterials, such as BaTiO3 nanocrsytal has been used a second harmonic (SH) probe for in vivo imaging [13]. These SHG nanoprobes are superior to the existing fluorescent dyes in terms of dye bleaching, signal saturation and narrow emission spectrum (full width at half maximum (FWHM)<5 nm) [13, 14]. Although a report showed that BaTiO3 nanocrsytals are non-toxic to zebrafish [13], inorganic nanomaterials have been questioned for their health and environmental issues which enormously limit their clinical applications [15-16].